The central nervous system can be manipulated via sensory pathways. Of interest here is a resonance method wherein periodic sensory stimulation evokes a physiological response that peaks at certain stimulus frequencies. This occurs for instance when rocking a baby, which typically provides relaxation at frequencies near 1/2 Hz. The peaking of the physiological response versus frequency suggests that one is dealing here with a resonance mechanism, wherein the periodic sensory signals evoke an excitation of oscillatory modes in certain neural circuits. The sensory pathway involved in the rocking example is the vestibular nerve. However, a similar relaxing response at much the same frequencies can be obtained by gently stroking a child's hair, or by administering weak heat pulses to the skin, as discussed in U.S. Pat. No. 5,800,481, Sep. 1, 1998. These three types of stimulation involve different sensory modalities, but the similarity in responses and effective frequencies suggests that the resonant neural circuitry is the same. Apparently, the resonance can be excited either via vestibular pathways or via cutaneous sensory pathways that carry tactile or temperature information.
Near 2.5 Hz another sensory resonance has been found that can be excited by weak heat pulses induced in the skin, as discussed in U.S. Pat. No. 5,800,481, Sep. 1, 1998. This sensory resonance brings on a slowing of certain cortical functions, as indicated by a pronounced increase in the time needed to silently count backward from 100 to 70 with the eyes closed. The effect is sharply dependent on frequency, as shown by a response peak a mere 0.13 Hz wide. The thermally excited 2.5 Hz resonance was found to also cause sleepiness, and after long exposure, dizziness and disorientation.
Other, more obscure types of stimulation in the form of weak magnetic fields or weak external electric fields can also cause the excitation of sensory resonances, as